Returning to Camelot, Agravaine once again seeks to bend Arthur's ear and questions Gaius's loyalty. He reminds Arthur that the old physician was once involved in the dark arts, adding, "Somebody is plotting against you and it is my duty to investigate every possibility!"
Meanwhile, Morgana pays a visit to the warrior priest Alator - he agrees to abduct Gaius in exchange for a special healing bracelet, forged on the Isle of the Blessed. Agravaine's plot gathers pace as Gaius is brought before the King...
He's questioned about his connection to the sorcerer that apparently killed Uther - Gaius claims that he'd heard of the man before the old King's death, but refuses to give up any further information.
Gaius departs, and Arthur grows suspicious, but refuses to condemn the old man without proof. "I fear that if we keep investigating… we might find some," says the slimy Agravaine.
Alator and a muscle-bound goon ride into Camelot, and sneak into the royal castle. With Merlin busy performing a task set by Agravaine, Gaius is alone and is snatched up by Alator. With the dark deed done, Agravaine sneaks into Gaius's chambers and plants a book on sorcery...
When it's noticed that Gaius is absent, Agravaine orders a search of his chambers, 'discovering' the magical texts he placed there. "There can be no doubt," he declares. "Gaius is the traitor!"
But Merlin tells Arthur that Gaius would never betray the kingdom. "Agravaine has made this story up!" he rants. "Because he's your Uncle, you will see who he really is!" But Arthur snaps back, "Gaius condemned himself - there's nothing more to be said!"
Alator delivers Gaius to Morgana, who warns her captive that he will be tortured for information. Soon enough, Alator uses his magic to torment Gaius with enchanted flame...
Merlin is determined to prove Agravaine's guilt and sneaks into his chambers, discovering a strange substance on his enemy's boots. Agravaine returns, and though Merlin is able to sneak out without being spotted, the villain senses that something is not right...
Merlin returns to his chamber to find Gwaine waiting - the warlock's old pal offers to help him track down Gaius! Gwaine is able to identify the substance Merlin found as a rare compound that only exists in a remote part of the kingdom. The pair ride out from the castle, but Agravaine has spotted them...
While Agravaine rushes to Morgana's hut to warn her of the coming danger, Gaius's torture continues. Alator demands to know the secret of Emrys, and - in great pain and under magical influence - Gaius groans, "I know him by a different name. His name is… Merlin!"
But Gaius doesn't stop there - in a delirious state, he tells Alator Merlin's great power and his destiny to bring about a time of peace, the time of Albion.
Merlin and Gwaine follow their trail inside a dark cave, but are confronted by Alator's goon. But the thug is soon dispatched, thanks to the nifty combination of Gwaine's swordplay and Merlin's magic.
Morgana and Agravaine arrive at the cave and she sends him to deal with Gaius, snarling, "I'll deal with Merlin and this hothead!"
Agravaine stalks towards an unconscious Gaius, prepares to slit his throat, but is caught in the act by Gwaine. However, the cunning villain claims that he was just using his blade to check if Gaius was still breathing - a likely story!
Agravaine claims that he followed Merlin and Gwaine to the cave, and insists that they must return the weakened Gaius to Camelot immediately to save his life. "Merlin can find his own way back!" he declares, and Gwaine reluctantly goes along with him.
But Morgana has caught up with Merlin and attacks him with magic. Alator appears at her side, and reveals: "Gaius told me everything - not only do I know who Emrys is, I know where he is."
But Alator will not tell Morgana - instead, he switches sides, using his magic to throw her against a stone pillar and knocking her out cold. Turns out Alator has been inspired by Gaius's words - he too has been hunted and persecuted for his magical gifts.
"I and others like me have dreamt of the world you seek to build, and we would gladly give our lives to help you do it!" says the warrior priest, kneeling before the young warlock.
Gaius is returned to Camelot, and to keep his own cover intact, Agravaine is forced to admit the old man's innocence. When Gaius wakes, he feels ashamed for having given up Merlin's secret. But Merlin is more worried about the scheming Agravaine - his guilt cannot be proven without hard evidence...
Arthur arrives to say sorry to both men, but Merlin insists that only Gaius deserves an apology. The King seems satisfied with that and promptly sets Merlin back to work - but we know they're good pals really!
Arthur is grateful for Gaius's safe return, but still wants to know why the old man wants to protect the sorcerer who killed Uther. Gaius tells him that the mysterious magician did not kill the old King - instead, he did everything in his power to save him,
Gaius suggests that many who use magic support Arthur and have much love for Camelot. "One day," he tells the young monarch, "you will understand just how much they've done for you."
• 'The Secret Sharer' doesn't break any new ground for Merlin - last week, the title character ended up in Morgana's clutches, this week it's Gaius. But the resolution is at least imaginative - rather than have Gaius hold out, he gives in to the torture, only to have Alator be so moved by his words that the former villain switches sides!
• It's great to see Gwaine play a slightly larger role this time round. He's one of our favourite Knights and has been relegated to a bit-player so far this series, so we're happy to see the charismatic Eoin Macken get more to do!
• Whatever happened to Arthur suspecting his uncle as the traitor? He fingered Agravaine last week, but here he won't listen to Merlin's accusations. Maybe he's just in denial?
• Lovely final scene between Arthur and Gaius - expertly played by Bradley James and Richard Wilson, who don't often get scenes alone together.